This invention relates generally to electromagnetic interference (EMI) TANK filter assemblies, particularly of the type used in active implantable medical devices (AIMDs) such as cardiac pacemakers, cardioverter defibrillators, neurostimulators and the like, which raise the impedance of internal electronic or related wiring components of the medical device at selected frequencies in order to reduce or eliminate currents induced from undesirable electromagnetic interference (EMI) signals. The invention is also applicable to a wide variety of commercial, telecommunications, military and space applications. The present invention is also applicable to a wide range of external medical devices, including externally worn drug pumps, EKG/ECG electrodes, neurostimulators, ventricular assist devices and the like. The present invention is also applicable to a wide range of probes, catheters, monitoring lead wires and the like that may be temporarily inserted into or onto a patient or that a patient may be wearing or connected to during medical diagnostic procedures such as MRI.
Compatibility of cardiac pacemakers, implantable defibrillators and other types of active implantable medical devices with magnetic resonance imaging (MRI) and other types of hospital diagnostic equipment has become a major issue. If one goes to the websites of the major cardiac pacemaker manufacturers in the United States, which include St. Jude Medical, Medtronic and Boston Scientific CRM (formerly Guidant), one will see that the use of MRI is generally contra-indicated with pacemakers and implantable defibrillators. A similar contra-indication is found in the manuals of MRI equipment manufacturers such as Siemens, GE, and Phillips. See also “Safety Aspects of Cardiac Pacemakers in Magnetic Resonance Imaging”, a dissertation submitted to the Swiss Federal Institute of Technology Zurich presented by Roger Christoph Lüchinger. “Dielectric Properties of Biological Tissues: I. Literature Survey”, by C. Gabriel, S. Gabriel and E. Cortout; “Dielectric Properties of Biological Tissues: II. Measurements and the Frequency Range 0 Hz to 20 GHz”, by S. Gabriel, R. W. Lau and C. Gabriel; “Dielectric Properties of Biological Tissues: III. Parametric Models for the Dielectric Spectrum of Tissues”, by S. Gabriel, R. W. Lau and C. Gabriel; and “Advanced Engineering Electromagnetics, C. A. Balanis, Wiley, 1989, all of which are incorporated herein by reference.
However, an extensive review of the literature indicates that MRI is indeed often used with pacemaker patients, in spite of the contra indications. The safety and feasibility of MRI in patients with cardiac pacemakers is an issue of gaining significance. The effects of MRI on patients' pacemaker systems have only been analyzed retrospectively in some case reports. There are a number of papers that indicate that MRI on new generation pacemakers can be conducted up to 0.5 Tesla (T). Other papers go up to 1.5 T for non-pacemaker dependent patients under highly controlled conditions. MRI is one of medicine's most valuable diagnostic tools. MRI is, of course, extensively used for imaging, but is also increasingly used for real-time procedures such as interventional medicine (surgery). In addition, MRI is used in real time to guide ablation catheters, neurostimulator tips, deep brain probes and the like. An absolute contra-indication for pacemaker patients means that pacemaker and ICD wearers are excluded from MRI. This is particularly true of scans of the thorax and abdominal areas. However, because of MRI's incredible value as a diagnostic tool for imaging organs and other body tissues, many physicians simply take the risk and go ahead and perform MRI on a pacemaker patient. The literature indicates a number of precautions that physicians should take in this case, including limiting the applied power of the MRI in terms of the specific absorption rate (SAR), programming the pacemaker to a fixed or asynchronous pacing mode, having emergency personnel and resuscitation equipment standing by (known as “Level II” protocol), and careful reprogramming and evaluation of the pacemaker and patient after the procedure is complete. There have been reports of latent problems with cardiac pacemakers after an MRI procedure occurring many days later (such as increase in or loss of pacing pulse capture).
There are three types of electromagnetic fields used in an MRI unit. The first type is the main static magnetic field designated B0 which is used to align protons in body tissue. The field strength varies from 0.5 to 3.0 Tesla in most of the currently available MRI units in clinical use. Some of the newer MRI system fields can go as high as 4 to 6 Tesla. At the recent International Society for Magnetic Resonance in Medicine (ISMRM), which was held on 5 and 6 Nov. 2005, it was reported that certain research systems are going up as high as 11.7 Tesla and will be ready sometime in 2007. A 1.5 T MRI system is over 100,000 times the magnetic field strength of the earth. A static magnetic field of this magnitude can induce powerful magnetomechanical forces on any magnetic materials implanted within the patient, including certain components within the cardiac pacemaker and/or lead wire systems themselves. It is unlikely that the static MRI magnetic field can induce currents (dB/dt) into the pacemaker lead wire system and hence into the pacemaker itself. It is a basic principle of physics that a magnetic field must either be time-varying as it cuts across the conductor (dB/dt), or the conductor itself must move within the magnetic field for currents to be induced (dB/dx).
The second type of field produced by magnetic resonance imaging equipment is the pulsed RF field which is generated by the body coil or head coil, also referred to as B1. This is used to change the energy state of the protons and illicit MRI signals from tissue. The RF field is homogeneous in the central region and has two main components: (1) the magnetic field is circularly polarized in the actual plane; and (2) the electric field is related to the magnetic field by Maxwell's equations. In general, the RF field is switched on and off during measurements and usually has a frequency of 21 MHz to 64 MHz to 128 MHz depending upon the static magnetic field strength. The frequency of the RF pulsed varies with the field strength of the main static field, as expressed in the Lamour Equation: RF PULSED FREQUENCY (in MHz)=(42.56) (STATIC FIELD STRENGTH (T); where 42.56 MHz per Tesla is the Lamour constant for H+ protons.
The third type of electromagnetic field is the time-varying magnetic gradient field designated Gx, y, z which is used for spatial localization. The gradient field changes its strength along different orientations and operating frequencies on the order of 1 to 2.2 kHz. The vectors of the magnetic field gradients in the X, Y and Z directions are produced by three sets of orthogonally positioned coils and are switched on only during the measurements. In some cases, the gradient field has been shown to elevate natural heart rhythms (heart beat). This is not completely understood, but it is a repeatable phenomenon. There have been some reports of gradient field induced ventricular arrhythmias which could be life threatening. However, in contrast, the gradient field is not considered by some researchers to create any significant adverse effects.
As previously stated, the third type of electromagnetic field is the time-varying magnetic gradient fields, designated as Gx, Gy, and Gz. The Gz gradient is used to distort the B0 field in the z direction, thereby creating body ‘slices’ of specific thickness. The Gx and Gy fields are used to introduce phase and frequency ‘markers’ to specific protons, allowing for an x-y image to be generated.
The fields operate at roughly 1 to 2.2 kHz, and are generated by three distinct, orthogonally oriented coils. These fields are only active during image generation protocols, and have been shown to have adverse effects on human physiology. These effects are largely due to the induced voltages that are generated by the application of a moving magnetic field on a large area. Following is Faraday's Law of Induction,
      V    =          A      ⁢                          ⁢                        ⅆ          B                          ⅆ          t                      ,Where A is the area of the loop, and dB/dt is change in magnetic flux with respect to time, it has been shown that the induced voltages generated by the gradient fields, if high enough, can induce peripheral nerve stimulation (PNS). This has been reported in literature as a sensation of pain or other discomfort while running relatively high MRI gradients. In more extreme animal testing, cardiac stimulation has been detected, although this has taken roughly 80 times more energy to achieve than that of PNS. To prevent PNS or cardiac stimulation from occurring, industry standards have limited dB/dt to roughly 20 T/sec.
Of interest is the effect of the gradient fields on AIMDs, which typically have implanted lead systems. In the case of AIMDs with unipolar lead systems, a circuit loop is formed between the AIMD housing, the lead system, the distal TIP, and body tissue (as the return path). An average area created by such a loop is around 225 cm2 with the higher limit about 350 cm2. When considering this with the 20 T/sec maximum, it can be seen that the maximum induced voltage in the loop is 0.700V. When one looks at the induced voltage at the pacing tip, it is typically an order of magnitude lower than the induced voltage in the loop (due to relatively high lead system and device impedances). This is much lower than the typical pacing threshold required for an AIMD to stimulate heart tissue.
It is instructive to note how voltages and EMI are induced into an implanted or external lead wire system. At very low frequency (VLF), voltages are induced at the input to the cardiac pacemaker as currents circulate throughout the patient's body and create differential voltage drops. In a unipolar system, because of the vector displacement between the pacemaker housing and, for example, the TIP electrode, voltage drop across body tissues may be sensed due to Ohms Law and the circulating RF signal. At higher frequencies, the implanted lead wire systems actually act as antennas where currents are induced along their length. These antennas are not very efficient due to the damping effects of body tissue; however, this can often be offset by extremely high power fields and/or body resonances. At very high frequencies (such as cellular telephone frequencies), EMI signals are induced only into the first area of the lead wire system (for example, at the header block of a cardiac pacemaker). This has to do with the wavelength of the signals involved and where they couple efficiently into the system.
Magnetic field coupling into an implanted lead wire system is based on loop areas. For example, in a cardiac pacemaker, there is a loop formed by the lead wire as it comes from the cardiac pacemaker housing to its distal TIP located in the right ventricle. The return path is through body fluid and tissue generally straight from the TIP electrode in the right ventricle back up to the pacemaker case or housing. This forms an enclosed area which can be measured from patient X-rays in square centimeters. The average loop area is 200 to 225 square centimeters. This is an average and is subject to great statistical variation. For example, in a large adult patient with an abdominal implant, the implanted loop area is much larger (greater than 450 square centimeters).
Relating now to the specific case of MRI, the magnetic gradient fields would be induced through enclosed loop areas. However, the pulsed RF fields, which are generated by the body coil, would be primarily induced into the lead wire system by antenna action.
There are a number of potential problems with MRI, including:
(1) Closure of the pacemaker reed switch. A pacemaker reed switch, which can also be a Hall Effect device, is designed to detect a permanent magnet held close to the patient's chest. This magnet placement allows a physician or even the patient to put the implantable medical device into what is known as the “magnet mode response.” The “magnet mode response” varies from one manufacturer to another, however, in general, this puts the pacemaker into a fixed rate or asynchronous pacing mode. This is normally done for short times and is very useful for diagnostic purposes. However, when a pacemaker is brought close to the MRI scanner, the MRI static field can make the pacemaker's internal reed switch close, which puts the pacemaker into a fixed rate or asynchronous pacing mode. Worse yet, the reed switch may bounce or oscillate. Asynchronous pacing may compete with the patient's underlying cardiac rhythm. This is one reason why pacemaker/ICD patients have generally been advised not to undergo MRI. Fixed rate or asynchronous pacing for most patients is not an issue. However, in patients with unstable conditions, such as myocardial ischemia, there is a substantial risk for life threatening ventricular fibrillation during asynchronous pacing. In most modern pacemakers the magnetic reed switch (or Hall Effect device) function is programmable. If the magnetic reed switch response is switched off, then synchronous pacing is still possible even in strong magnetic fields. The possibility to open and re-close the reed switch in the main magnetic field by the gradient field cannot be excluded. However, it is generally felt that the reed switch will remain closed due to the powerful static magnetic field. It is theoretically possible for certain reed switch orientations at the gradient field to be capable of repeatedly closing and re-opening the reed switch.(2) Reed switch damage. Direct damage to the reed switch is theoretically possible, but has not been reported in any of the known literature. In an article written by Roger Christoph Lüchinger of Zurich, he reports on testing in which reed switches were exposed to the static magnetic field of MRI equipment. After extended exposure to these static magnetic fields, the reed switches functioned normally at close to the same field strength as before the test.(3) Pacemaker displacement. Some parts of pacemakers, such as the batteries and reed switch, contain ferrous magnetic materials and are thus subject to mechanical forces during MRI (testing is done to ASTM Standards). Pacemaker displacement may occur in response to magnetic force or magnetic torque (newer pacemakers and ICDs have less ferrous materials and are less susceptible to this).(4) Radio frequency field. At the pulsed RF frequencies of interest in MRI, RF energy can be absorbed and converted to heat. The power deposited by RF pulses during MRI is complex and is dependent upon the power. duration and shape of the RF pulse, the relative long term time averages of the pulses, the transmitted frequency, the number of RF pulses applied per unit time, and the type of configuration of the RF transmitter coil used. Specific absorption rate (SAR) is a measure of how much energy is induced into body tissues. The amount of heating also depends upon the volume of the various tissue (i.e. muscle, fat, etc.) imaged, the electrical resistivity of tissue and the configuration of the anatomical region imaged. There are also a number of other variables that depend on the placement in the human body of the AIMD and its associated lead wire(s). For example, it will make a difference how much current is induced into a pacemaker lead wire system as to whether it is a left or right pectoral implant. In addition, the routing of the lead and the lead length are also very critical as to the amount of induced current and heating that would occur. Also, distal TIP design is very important as the distal TIP itself can act as its own antenna. Location within the MRI bore is also important since the electric fields required to generate the RF increase exponentially as the patient is moved away from MRI bore center-line (ISO center). The cause of heating in an MRI environment is two fold: (a) RF field coupling to the lead can occur which induces significant local heating; and (b) currents induced during the RF transmission can flow into body tissue and cause local Ohm's Law heating next to the distal TIP electrode of the implanted lead. The RF field in an MRI scanner can produce enough energy to induce lead wire currents sufficient to destroy some of the adjacent myocardial tissue. Tissue ablation has also been observed. The effects of this heating are not readily detectable by monitoring during the MRI. Indications that heating has occurred would include an increase in pacing threshold, venous ablation, Larynx ablation, myocardial perforation and lead penetration, or even arrhythmias caused by scar tissue. Such long term heating effects of MRI have not been well studied yet.(5) Alterations of pacing rate due to the applied radio frequency field. It has been observed that the RF field may induce undesirable fast cardiac pacing (QRS complex) rates. There are various mechanisms which have been proposed to explain rapid pacing: direct tissue stimulation, interference with pacemaker electronics or pacemaker reprogramming (or reset). In all of these cases, it would be desirable to raise the lead system impedance (to reduce RF current), make the feedthrough capacitor more effective and provide a very high degree of protection to AIMD electronics. This will make alterations in pacemaker pacing rate and/or pacemaker reprogramming much more unlikely.(6) Time-varying magnetic gradient fields. The contribution of the time-varying gradient to the total strength of the MRI magnetic field is negligible, however, pacemaker systems could be affected because these fields are rapidly applied and removed. The time rate of change of the magnetic field is directly related to how much electromagnetic force (EMF) and hence current can be induced into a lead wire system. Lüchinger reports that even using today's gradient systems with a time-varying field up to 60 Tesla per second, the induced currents are likely to stay below the biological thresholds for cardiac fibrillation. A theoretical upper limit for the induced voltage by the time-varying magnetic gradient field is 20 volts. Such a voltage during more than 0.1 milliseconds could be enough energy to directly pace the heart.(7) Heating. Currents induced by time-varying magnetic gradient fields may lead to local heating. Researchers feel that the calculated heating effect of the gradient field is much less as compared to that caused by the RF field and therefore may be neglected.
There are additional problems possible with implantable cardioverter defibrillators (ICDs). ICDs use different and larger batteries which could cause higher magnetic forces. The programmable sensitivity in ICDs is normally much higher than it is for pacemakers, therefore, ICDs may falsely detect a ventricular tacchyarrhythmia and inappropriately deliver therapy. In this case, therapy might include anti-tacchycardia pacing, cardio version or defibrillation (high voltage shock) therapies. MRI magnetic fields may prevent detection of a dangerous ventricular arrhythmia or fibrillation. There can also be heating problems of ICD leads which are expected to be comparable to those of pacemaker leads. Ablation of vascular walls is another concern. There have also been reports of older model ICDs being severely effected by the MRI pulsed RF field. In these cases, there have been multiple microprocessor resets and even cases of permanent damage where the ICD failed to function after the MRI procedure. In addition, ICDs have exhibited a different type of problem when exposed to MRI fields. That is, during an MRI exposure, the ICD might inappropriately sense the MRI RF—field or gradient fields as a dangerous ventricular arrhythmia. In this case, the ICD will attempt to charge its high energy storage capacitor and deliver a high voltage shock to the heart. However, within this charging circuit, there is a transformer that is necessary to function in order to fully charge up the high energy storage capacitor. In the presence of the main static field (B0) field, the ferrite core of this transformer tends to saturate thereby reducing its efficiency. This means the high energy storage capacitor cannot fully charge. Reports of repeated low voltage shocks are in the literature. These repeated shocks and this inefficient attempt to charge the battery can cause premature battery depletion of the ICD. Shortening of battery life is of course, a highly undesirable condition.
In summary, there are a number of studies that have shown that MRI patients with active implantable medical devices, such as cardiac pacemakers, can be at risk for potential hazardous effects. However, there are a number of anecdotal reports that MRI can be safe for extremity imaging of pacemaker patients (i.e. the AIMD is outside the bore). These anecdotal reports are of interest; however, they are certainly not scientifically convincing that all MRI can be safe. As previously mentioned, just variations in the pacemaker lead wire length can significantly effect how much heat is generated. From the layman's point of view, this can be easily explained by observing the typical length of the antenna on a cellular telephone compared to the vertical rod antenna more common on older automobiles. The relatively short antenna on the cell phone is designed to efficiently couple with the very high frequency wavelengths (approximately 950 MHz) of cellular telephone signals. In a typical AM and FM radio in an automobile, these wavelength signals would not efficiently couple to the relatively short antenna of a cell phone. This is why the antenna on the automobile is relatively longer. An analogous situation exists on the MRI system. If one assumes, for example, a 3.0 Tesla MRI system, which would have an RF pulsed frequency of 128 MHz, there are certain exact implanted lead lengths that would couple efficiently as fractions of the 128 MHz wavelength. Ignoring the effects of body tissue, as an example, the basic wavelength equation in meters is 300 divided by the frequency in MHz. Accordingly, for a 3.0 Tesla MRI system, the wavelength is 2.34 meters or 234 centimeters. An exact ¼ wavelength antenna then would be ¼ of this which is 58.59 centimeters. This falls right into the range for the length of certain pacemaker lead wire implants. It is typical that a hospital will maintain an inventory of various leads and that the implanting physician will make a selection depending on the size of the patient, implant location and other factors. Accordingly, the implanted or effective lead wire length can vary considerably. Another variable has to do with excess lead wire. It is typical that the physician, after a pacemaker lead wire insertion, will wrap up any excess lead wire in the pectoral pocket. This can form one, two or even three turns of excess lead. This forms a loop in that specific area, however, the resulting longer length of wire that goes down into the right ventricle, is what would then couple efficiently with the MRI RF pulsed frequency. As one can see, the amount of unwound up lead length is considerably variable depending upon patient geometry. There are certain implanted lead wire lengths that just do not couple efficiently with the MRI frequency and there are others that would couple very efficiently and thereby produce the worst case for heating. The actual situation for an implanted lead wire is far more complex due to the varying permittivity and dielectric properties of body tissues and the accompanying shifts in wavelengths.
The effect of an MRI system on the function of pacemakers, ICDs and neurostimulators depends on various factors, including the strength of the static magnetic field, the pulse sequence (gradient and RF field used), the anatomic region being imaged, and many other factors. Further complicating this is the fact that each manufacturer's pacemaker and ICD designs behave differently. Most experts still conclude that MRI for the pacemaker patient should not be considered safe. Paradoxically, this also does not mean that the patient should not receive MRI. The physician must make an evaluation given the pacemaker patient's condition and weigh the potential risks of MRI against the benefits of this powerful diagnostic tool. As MRI technology progresses, including higher field gradient changes over time applied to thinner tissue slices at more rapid imagery, the situation will continue to evolve and become more complex. An example of this paradox is a pacemaker patient who is suspected to have a cancer of the lung. RF ablation treatment of such a tumor may require stereotactic imaging only made possible through real time fine focus MRI. With the patient's life literally at risk, and with informed patient consent, the physician may make the decision to perform MRI in spite of all of the previously described attendant risks to the pacemaker system.
Insulin drug pump systems do not seem to be of a major current concern due to the fact that they have no significant antenna components (such as implanted lead wires). However, implantable pumps presently work on magneto-peristaltic systems, and must be deactivated prior to MRI. There are newer (unreleased) systems that would be based on solenoid systems which will have similar problems.
It is clear that MRI will continue to be used in patients with an active implantable medical device. There are a number of other hospital procedures, including electrocautery surgery, lithotripsy, etc., to which a pacemaker patient may also be exposed. Accordingly, there is a need for circuit protection devices which will improve the immunity of active implantable medical device systems to diagnostic procedures such as MRI.
As one can see, many of the undesirable effects in an implanted lead wire system from MRI and other medical diagnostic procedures are related to undesirable induced currents in the lead wire system. This can lead to overheating either in the lead wire or at the tissue interface at the distal TIP. At the 2006 SMIT Conference, the FDA reported on a neurostimulator patient whose implanted leads were sufficiently heated that severe burns occurred resulting in the need for multiple amputations. In pacemaker patients, these currents can also directly stimulate the heart into sometimes dangerous arrhythmias. The above descriptions of problems that a pacemaker, ICD or neurostimulator patients may encounter during MRI or similar medical diagnostic procedures are only examples of a general need. A patient wearing external devices, such as an external drug pump, an external neurostimulator, EKG leads, (skin patches) or ventricular assist devices, may also encounter problems during an MRI procedure. All of the above descriptions regarding overheating of lead wires, overheating of distal tips or electromagnetic interference are all concerns. The novel resonant TANK filter of the present invention is equally applicable to all of these other devices. It is also applicable to probes and catheters that are used during certain real time medical imaging procedures such as MRI. The present invention is applicable to a wide range of both implanted and external medical device systems. In general, the present invention is a circuit protection device that protects a patient undergoing high RF power medical diagnostic procedures.
Accordingly, there is a need for a novel resonant EMI TANK filter assembly which can be placed at various locations along the medical device lead wire system, which also prevents current from circulating at selected frequencies of the medical therapeutic device. Preferably, such novel TANK filters would be designed to resonate at or near 64 MHz for use in an MRI system operating at 1.5 Tesla (or 128 MHz for a 3 Tesla system), and have broad application to other fields, including telecommunications, military, space and the like. The present invention fulfills these needs and provides other related advantages.